The magnetic recording medium is widely used as audio tapes for recording sound, video tapes, or tapes or floppy disks for recording computer data. The magnetic recording medium of these types has on a support a magnetic layer containing a ferromagnetic powder dispersed in a binder.
The magnetic recording medium is required to be on high levels with respect to various characteristics, including electromagnetic characteristics, running durability and running performance. More specifically, the audio tapes for reproducing recorded music are required to have a higher level of original sound-reproducing ability; while the video tapes are required to have excellent electromagnetic characteristics, including an excellent ability to reproduce originals.
For the recording medium which possesses excellent electromagnetic characteristics, as described above, it is necessary at the same time to have good running durability. In general, the magnetic recording medium can attain good running durability by the addition of abrasives and lubricants to their magnetic layers.
For achieving excellent running durability by the use of abrasives, however, a measure of increase in their addition amount is required to result in reduction of the packing degree of a ferromagnetic powder in the magnetic layer. In a case where the abrasives having large grain sizes are used for the aforementioned purpose, on the other hand, they are apt to protrude to excess from the magnetic layer surface. Therefore, an improvement of running durability due to use of abrasives often causes deterioration in the above-described electromagnetic characteristics.
In a case where lubricants are used for improvement of the aforementioned running durability, a large amount of the lubricants added is required. As a result, the magnetic layer is subject to deterioration in durability because the binder comes to have a tendency to be plasticized.
Further, it is a matter of course that the binder as a main component of the magnetic layer performs an important function in effecting improvements in both the foregoing durability and electromagnetic characteristics. Conventional binders, such as polyvinylchloride resins, cellulose resins, polyurethane resins and acrylic resins, have a problem in that the magnetic layers using them are inferior in abrasion resistance to result in soiling the parts of a magnetic tape-running system such as guide poles or guide rolles.
As one approach to the solution of such a problem, it was proposed to raise the hardness of a magnetic layer by the use of a hard binder.
For instance, the magnetic recording medium using the binder comprising a polyesterpolyurethane resin and a polycarbonatepolyurehtane resin is described in JP-A-06-96437 (the term "JP-A" as used herein means an "unexamined published Japanese patent application"). In Examples in this reference, the binders in which the content of urethane groups is from 2 to 4 mmol/g are described, but the long-chain diol (called "second diol" hereinafter) contents therein are indefinite and the OH contents also are unspecified. In addition, JP-A-06-19821 describes a binder containing an urethaneurea in which the total content of urethane and urea is from 1.8 to 3.0 mmol/g. According to Synthesis Example of the binder, the proportion of second diols in the polyurethane resin obtained is 61 weight % and the urethane linkage density is high. Accordingly, such a binder ensures excellent durability, but has a disadvantage in that it causes a viscosity increase of the coating solution which is accompanied by the lowering of dispersibility to deteriorate the electromagnetic characteristics.
Furthermore, it was proposed to employ as the binder a polyurethane resin using a short-chain diol (called "first diol" hereinafter) having at least one cyclic hydrocarbon group. For instance, JP-A-61-148626 uses as a diol the polyesterpolyol containing bisphenol A in a proportion of 20%. By the calculation from the data in Example of the reference cited, the proportions of bisphenol A and the polyol in the polyurethane resin are evaluated to be 13 weight % and 69 weight %, respectively. Since the cyclic hydrocarbon groups in a resin lower the resin's solubilities in solvents, the polyurethane resin of the foregoing type has a drawback of being inferior in dispersibility. As another example of a polyurethane resin using a first diol having cyclic hydrocarbon groups, the polyurethane using bisphenol A as a chain-lengthening agent and the polycarbonatepolyol as a starting material is described in JP-A-01-251416. The proportions of bisphenol A and the polyol in such a polyurethane resin are evaluated to be 16 weight % and 63 weight %, respectively, by the calculation from the data in Example of the above-cited reference, and so this polyurethane resin also has a problem that its dispersibility is lowered due to the solubility-lowering effect of the cyclic hydrocarbon groups present therein. As a further examples of a polyurethane resin of the foregoing type, the polyurethane resin using a bisphenol S-containing and lactone-modified polyol is described in JP-B-07-21851 (the term "JP-B" as used herein means an "examined Japanese patent publication"). Therein, the proportions of the polyol and bisphenol S are evaluated to be 52 weight % and 13 weight %, respectively, by the calculation from the data in Example of the above-cited reference. Accordingly, such a polyurethane resin also has the same problem as mentioned above because of the presence of the cyclic hydrocarbon groups therein.
Also, the polyurethane resins using polyetherpolyols having cyclic hydrocarbon groups, e.g., the diols (molecular weight: 250 to 3,000) such as ethylene oxide or propylene oxide adducts of bisphenol A or hydrogenated bisphenol A, are described in U.S. Pat. No. 5,153,071. All the polyurethane resins obtained in Examples of this reference contain polyols in a proportion of no lower than 70 weight % and have an ether content of at least 8 mmol/g, so that the coated film made are soft. As a result, those coated film suffer from disadvantages of soiling a magnetic head and having low durability.
Further, JP-A-61-190717 describes the polyurethane resins using polytetramethyleneglycol and polycaprolactonepolyol. According to the descriptions in Examples of this reference, the polyols are present in each resin in a proportion of at least 70 weight %, so that the coated film formed are soft similarly to the above. Thus, such resins also have problems of soiling a magnetic head and deteriorating the durability.
Furthermore, JP-B-06-64726 describes the polyurethane resin obtained by reacting a branched polyesterpolyol with a prepolymer having terminal isocyanate groups. The content of OH groups in such a resin is evaluated to be 8.2.times.10.sup.-5 eq/g from the data in Synthesis Example. Such a high content of OH groups causes an increase in viscosity of the resin solution to lower the dispersibility. In addition, such a resin is attended by lowering of strength and deterioration in repeated running properties due to the presence of branched polyols therein.
Similarly to the above, the magnetic recording medium using a binder prepared from a compound having at least one OH group at both terminals and a polyisocyanate is described in U.S. Pat. No. 5,254,404. Although this reference describes only use of a polyesterpolyol, the binder obtained still has the resin strength-lowering problem and the problem of causing deterioration in repeated running properties.
In addition, JP-A-62-82510 describes the binder comprising a polyurethane resin wherein the number of terminal groups present on the main and branched chains is at least 3 on the average and primary hydroxyl groups are present on at least two terminals. In Examples thereof, use of polyesterpolyols is described. However, the resins prepared therein are insufficient in resin strength and repeated running durability.
In preparing conventionally known polyurethane resins and polyurethaneurea resins used as binders for a magnetic recording medium, as mentioned above, the second diols having hydrophilic segments, such as polyester, polyether and polycarbonate, are generally employed. According to Examples described in the prior arts, all the resins obtained contain the second diols in a proportion of at least 25 mole %.
However, the hydrophilic segments in the above-recited polyurethane resins and polyurethaneurea resins hinder the resins from having affinity for organic solvents, and the hydrophilic polar groups are subject to aggregation; as a result, the resins show a weak tendency to stretch their molecular chains in an organic solvent. Thus, the presence of hydrophilic segments in the resins acts adversely on the dispersion of a ferromagnetic fine powder.
In addition, those second diols having hydrophilic segments are attended by drawbacks that, when they are polyesters, the polyurethane resins obtained have inferior storage stability because of ester linkage's liability to hydrolysis; while, when they are polyethers such as polytetramethylene ether glycol, polypropylene glycol and polyethylene glycol, the polyurethane resins obtained are soft and have low Tg to form films having low strength.
Moreover, magnetic recording medium of another type is known, wherein the magnetic layer has a reduced thickness due to the formation on a nonmagnetic layer. In order to effect a further increase in recording density, it becomes necessary to further reduce the thickness of a magnetic layer and use a more finely pulverized ferromagnetic metal powder. The fine pulverization of a ferromagnetic metal powder causes the lowering of dispersibility to result in deterioration in surface quality and electromagnetic characteristics of the magnetic layer, and further to make it difficult for the magnetic layer to secure the durability.
In the present situation that it is required for a polyurethane resin to have excellent ability to disperse a ferromagnetic metal powder and a nonmagnetic powder, and to impart both hardness (high Tg and high Young's modulus) and elasticity (elongation) to the magnetic layer, and to ensure excellent durability in the magnetic layer, the polyurethane resins as recited above fail in satisfactorily meeting the aforesaid requirements.
Although the tapes utilizing vacuum deposited thin films of ferromagnetic metals (ME) are available at present as 8-mm video tapes and video tapes appropriate to DVC (which stands for a digital video camera) for consumer use, ferromagnetic metal powder-coated tapes (MP) have an advantage of being superior to ME in durability and economic efficiency, but on the other hand they are inferior to ME in electromagnetic characteristics.
And there is a desire to make further improvements in the running durability and the electromagnetic characteristics of magnetic fine powder-coated magnetic recording medium.